Of course there are also hundreds of other factors deciding the outcome, so Facebook isn't special in any way.
So it's perfectly possible to feed 10000 voters their personally crafted propaganda and have nobody else even aware that it is happening.
1. You'd have to know that 10 000 votes is missing.
2. You'd have to know how to target those 10 000 missing voters (criteria).
3. You'd have to craft very specific messages based on your criteria to determine who they are.
4. Now, hoping that those criteria are fit you have to hope that the message you crafted is going to be seen and noticed by your prospects.
5. Message noticed ? Good, how did you craft a message that would make up many voter's mind without being obvious ?
6. Who cares, you just have to convince the team running the campaign that social network marketing converts. That's the real skill.